Dextrorphan is a selective noncompetitive antagonist of the N-methyl- D-aspartate (NMDA) subfamily of glutamate receptors. Like other ligand-activated ion channels, the NMDA receptor appears to be composed of several subunits which co-assemble to form a functional channel. The specific aims of the projected research are to (1) identify the subunit of the NMDA receptor with which [3H]dextrorphan interacts and (2) to elucidate the specific structural domains responsible for the interaction of [3H]dextrorphan with the NMDA receptor-operated ion channel. Radioligand binding studies will be used to characterize [3H]dextrorphan binding in membrane preparations derived from COS cells transiently transfected with wild-type and mutant recombinant NMDA receptor constructs. In addition, immunologic techniques will be used to confirm both expression and co-assembly of the various subunits in COS cell membranes. Although the research goals of this project are to characterize the structural determinants responsible for the interaction of [3H]dextrorphan with recombinant NMDA receptors, an overall objective of the proposed research is to provide a more complete mechanistic understanding of the functional significance of various combinations of NMDA receptor subunits. In turn, these studies may provide useful insights into the molecular basis for the psychotomimetic properties of noncompetitive NMDA antagonists as well as the therapeutic potential of these compounds to reduce neuronal injury associated with stroke, head trauma, hypoglycemia and sustained convulsions.